The present invention relates to electrostatically actuated devices and more particularly to silicon-based actuators, which are used, for example, in ink-jet printing, fluid pumping and optical switching, for preventing electrical breakdown in electrostatically actuated devices.
In ink-jet printing, droplets of ink are selectively ejected from a plurality of drop ejectors in a print head. The ejectors are operated in accordance with digital instructions to create a desired image on a print medium moving past the print head. The print head may move back and forth relative to the sheet in a typewriter fashion, or in the linear array may be of a size extending across the entire width of a sheet, to place the image on a sheet in a single pass.
The ejectors typically comprise actuators connected to both a nozzle or drop ejection aperture and to one or more common ink supply manifolds. Ink is retained within each channel until there is a response by the actuator to an appropriate signal. In one embodiment of the ejector, the ink drop is ejected by the pressure transient due to volume displacement of an electrostatically or magnetostatically actuated deformable membrane, which typically is a capacitor structure with a flexible electrode, fixed counter electrode, and actuated by a voltage bias between the two electrodes.
Silicon-based actuators also can be employed in micro-electromechanical devices that can be used for pumping and switching, and wherein for example, silicon based actuators are, respectively, used for microfluid pumping, and optical switching. Fluids are pumped due to the volume displacement of an electrostatically or magnetostatically deformable membrane, which is a capacitor structure with a flexible electrode, fixed counter electrode, and actuated by a voltage bias between the two silicon electrodes. Optical switching occurs by the displacement of optical elements as a result of actuation due to electrostatic or magnetostatic interactions with other on-chip elements or magnetostatic device package. For example, in optical switching a mirror can be employed as the optical element using electrostatic actuators to provide the displacement.
This capacitor structure which incorporates a deformable membrane for these silicon-based actuators can be fabricated in a standard polysilicon surface micro-machining process. It can be batch fabricated at low cost using existing silicon foundry capabilities. The surface micro-machining process has proven to be compatible with integrated microelectronics, allowing for the monolithic integration of the actuation with associated addressing electronics.
A problem associated with capacitive-based electrostatic actuators, and in particular the above-described silicon-based actuators, is the undesirable electrode damage resulting from contact and arcing between the electrodes of the micro-electro-mechanical structure (MEMS), which is observed after a few thousand cycles. The thin silicon membranes will flex enough to allow inter-electrode contact at some point between the device center, which is supported and prevented from contact, and the edge of the membrane, which is also spaced from the counter-electrode. The contact and subsequent arcing at these intermediate points are linked to early degradation and failure of the capacitor structure, significantly shortening the life of the actuator.
It is an object of the invention to prevent membranes with large surface area from contacting the counter-electrode.
There is provided an electrostatic device including a semiconductor substrate with an insulating layer, or an insulating substrate, a conductor on the insulating layer, a flexible membrane with its sides supporting itself above the conductor (counter-electrode), and an actuator chamber formed between the membrane and the opposing counter-electrode. A power source is connected between the counter-electrode and the membrane and when activated the resulting force is sufficient to deflect the flexible membrane toward the counter-electrode, thereby increasing the supply of fluid in a chamber above the membrane. As the power source is removed, the membrane will return to its equilibrium position, thus pressurizing the fluid in the chamber above the movable membrane. The addition of the structures between the membrane and counter-electrode, which act to prevent the two electrodes from touching due to excessive flexure of the membrane, prevents arcing and subsequent damage when the device is actuated.
These and other aspects of the invention will become apparent from the following description, the description being used to illustrate a preferred embodiment of the invention when read in conjunction with the accompanying drawings.